Ignorance is when you don’t know something. Stupidity is when you think you know something and are presented with evidence to the contrary, but you dismiss it because of something stupid (i.e., irrational).

Of course, all these words have some overlap. My larger point is, people rarely come to rational conclusions organically, and it takes decades to centuries for even the most empirically verifiable idea to permeate, especially in the face of misinformation campaigns or when against “common sense”.